An atom is neutral when it has an equal number of electrons and protons.
Let's analyze the given options:
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An oxygen atom with 16 electrons, 18 protons, and 16 neutrons:
- Electrons: 16
- Protons: 18
- Not neutral (more protons than electrons).
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An oxygen atom with 9 electrons, 8 protons, and 8 neutrons:
- Electrons: 9
- Protons: 8
- Not neutral (more electrons than protons).
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An oxygen atom with 8 electrons, 8 protons, and 9 neutrons:
- Electrons: 8
- Protons: 8
- Neutral (equal number of electrons and protons).
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An oxygen atom with 4 electrons, 6 protons, and 4 neutrons:
- Electrons: 4
- Protons: 6
- Not neutral (more protons than electrons).
The neutral atom is: An oxygen atom with 8 electrons, 8 protons, and 9 neutrons.